Carrier aggregation is one of the new features recently developed by the members of the 3rd-Generation Partnership Project (3GPP) for so-called Long Term Evolution (LTE) systems, and is standardized as part of LTE Release 10, which is also known as LTE-Advanced. An earlier version of the LTE standards, LTE Release 8, supports bandwidths up to 20 MHz. In LTE-Advanced, bandwidths up to 100 MHz are supported. The very high data rates contemplated for LTE-Advanced will require an expansion of the transmission bandwidth. In order to maintain backward compatibility with LTE Release 8 mobile terminals, the available spectrum is divided into Release 8—compatible chunks called component carriers. Carrier aggregation enables bandwidth expansion beyond the limits of LTE Release 8 systems by allowing mobile terminals to transmit data over multiple component carriers, which together can cover up to 100 MHz of spectrum. Importantly, the carrier aggregation approach ensures compatibility with earlier Release 8 mobile terminals, while also ensuring efficient use of a wide carrier by making it possible for legacy mobile terminals to be scheduled in all parts of the wideband LTE-Advanced carrier.
The number of aggregated component carriers, as well as the bandwidth of the individual component carrier, may be different for uplink (UL) and downlink (DL) transmissions. A carrier configuration is referred to as “symmetric” when the number of component carriers in each of the downlink and the uplink are the same. In an asymmetric configuration, on the other hand, the numbers of component carriers differ between the downlink and uplink. The number of component carriers configured for a geographic cell area may be different from the number of component carriers seen by a given mobile terminal. A mobile terminal, for example, may support more downlink component carriers than uplink component carriers, even though the same number of uplink and downlink component carriers may be offered by the network in a particular area.
LTE systems can operate in either Frequency-Division Duplex (FDD) mode or Time-Division Duplex (TDD) mode. In FDD mode, downlink and uplink transmissions take place in different, sufficiently separated, frequency bands. In TDD mode, on the other hand, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum. TDD mode also allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. These differing configurations permit the shared frequency resources to be allocated to downlink and uplink use in differing proportions. Accordingly, uplink and downlink resources can be allocated asymmetrically for a given TDD carrier.
One consideration for carrier aggregation is how to transmit control signaling from the mobile terminal on the uplink to the wireless network. Uplink control signaling may include acknowledgement (ACK) and negative-acknowledgement (NACK) signaling for hybrid automatic repeat request (Hybrid ARQ, or HARQ) protocols, channel state information (CSI) and channel quality information (CQI) reporting for downlink scheduling, and scheduling requests (SRs) indicating that the mobile terminal needs uplink resources for uplink data transmissions.
Another element of uplink transmissions in LTE systems is the Sounding Reference Signal (SRS), which can be viewed as a type of pilot signal sent by an LTE mobile terminal (commonly called “user equipment,” or “UE,” in 3GPP documentation. Because the receiving base station (an “eNodeB,” or “eNB,” in 3GPP terminology) knows what the received SRS should look like, it can estimate the channel according to this formula:Received signal=Sent signal*Channel+Noise
The eNB can use the channel estimate obtained from SRS to perform link adaptation, to select an appropriate modulation and coding scheme (MCS) for the UE, etc.
In LTE, there are two types of SRS. Aperiodic SRS is sent by the UE upon request from the eNB. Periodic SRS is sent by the UE periodically. The periodic SRS is beneficial compared to the aperiodic SRS in that the eNB does not need to specifically request each periodic SRS transmission.
Channel Quality Indicator (CQI) refers to a measurement of downlink reference symbols, performed by a UE, and the resulting report sent to the eNB. The measurements may be made on Cell-specific Reference signals (CRS) and/or UE-specific reference signals. The UE knows the CRS signal and/or UE-specific reference signals sent by the eNB and estimates the effects of the downlink channel, again according to the basic formula:Received signal=Sent signal*Channel+Noise
The UE reports the channel estimation results back to the eNB and the eNB can use the information to perform link adaptation, select an appropriate modulation and coding scheme (MCS) for the UE etc. Note that in LTE, the reported channel estimate information is referred to as Channel State Information (CSI) or a channel-state report, which is a combination of a Channel Quality Indicator (CQI), which indicates the highest modulation-and-coding scheme that could be used in the downlink while maintaining a target block-error rate, a Rank Indicator, which indicates a transmission rank to use, and a precoder matrix indicator (PMI), which recommends a precoding matrix to be used for multi-layer transmission.
A time-alignment or timing-advance (TA) mechanism has been introduced in LTE to ensure that the uplink signals from different UEs are received by the network receiver in a time-aligned fashion. The UE maintains a TA value that tells the UE how much it should advance its uplink signals in relation to a timing reference. The eNB then sends TA commands to each UE, ordering them to transmit their UL signals earlier or later, i.e., to decrease or increase their TA value respectively, so that all UE's signals reach the receiving eNB time-aligned.
The UE has a TA timer that controls the validity of the TA value. If the TA timer is running, then the TA value is considered valid and the UE is allowed to perform UL transmissions. When the UE applies a received TA command it restarts the TA timer which means that the period for which the TA value is valid is extended. When the TA timer expires the UE is considered out-of-sync and is not allowed to perform uplink transmissions.
To support uplink carrier aggregation of uplink cells received at different reception points, Release 11 of the LTE specifications introduces multiple TA values. The concept of TA groups (TAGs) is also introduced. Each TA group has a TA value, a TA timer and a timing reference (the timing reference is the downlink reception timing of a cell within the TA group). A UE's cells may be grouped together in the TA groups according to which reception point is receiving the uplink transmissions for the cells, for example. The TAG containing the primary cell (PCell) is called the PCell TA group, or pTAG, but can contain one or more secondary cells (SCells). There can also be up to four other TA groups for a UE, each including only secondary cells, and which are called SCell TA groups, or sTAGs. Which TA group a UE's serving cells belong to is decided by the eNB and signaled by RRC signaling to the UE.
Since the PCell is “always on” and is needed for the UE to maintain connection to the network, it is expected that the pTAG's TA timer needs to always run when the UE is in connected state. sTAGs, however, only contain SCells, which are additional resources and are therefore less important.
The UE receives the configuration for periodic SRS and periodic CQI from the eNB through Radio Resource Control (RRC) signaling. RRC signaling is comparatively slow, relative to Medium Access Control (MAC) signaling, and within 3GPP it has been targeted that RRC messages should be sent, if possible, less than once per second. Accordingly, there are no mechanisms to permit the eNB to stop periodic SRS/CQI signaling on a short timescale.